1. Field of the Invention
The present invention relates to a fuel cell stack comprising separators and fuel cell units each including an anode electrode and a cathode electrode with an electrolyte interposed therebetween, the fuel cell units and the separators being alternately stacked.
2. Description of the Related Art
For example, the solid polymer fuel cell is generally composed of a fuel cell stack comprising fuel cell units stacked with each other by being interposed by separators, each of the fuel cell units including an anode electrode and a cathode electrode which are arranged on both sides of an electrolyte composed of a polymer ion exchange membrane (cation exchange membrane).
In such a fuel cell stack, the fuel gas, for example, a gas principally containing hydrogen (hereinafter referred to as xe2x80x9chydrogen-containing gasxe2x80x9d), which is supplied to the anode electrode, contains hydrogen which is ionized on the catalyst electrode and which is moved to the cathode electrode via the electrolyte that is appropriately humidified. The electron, which is generated during this process, is extracted by an external circuit, and it is utilized as DC electric energy. The cathode electrode is supplied with an oxidizing gas, for example, a gas principally containing oxygen (hereinafter referred to as xe2x80x9coxygen-containing gasxe2x80x9d) or air. Therefore, the hydrogen ion, the electron, and the oxygen are reacted with each other on the cathode electrode to produce water.
In such a process, it is necessary to sufficiently humidify the electrolyte composed of the polymer ion exchange membrane in order to maintain the ion permeability. For this reason, the fuel cell is generally constructed as follows. That is, the oxygen-containing gas and the fuel gas are humidified by using a gas-humidifying unit which is provided at the outside of the fuel cell. The gases are fed to the fuel cell stack together with water vapor so that the electrolyte is humidified.
The fuel cell stack comprises the plurality of fuel cell units which are stacked together with the separators intervening therebetween. Communication holes, which are used to supply the fuel gas, the oxygen-containing gas, and the cooling water (cooling medium) to the respective fuel cell units, are formed to penetrate through the plurality of fuel cell units in an integrated manner in the fuel cell stack. In this arrangement, it is necessary that the fluid such as the fuel gas is uniformly supplied to the respective fuel cell units stacked in the fuel cell stack, in order to maintain the power generation performance of each of them.
In this context, a fuel cell is known, which is disclosed, for example, in Japanese Laid-Open Patent Publication No. 8-213044. This conventional technique adopts delivery flow passages for delivering the fuel which is allowed to flow thereinto from an inflow port, to each of a plurality of unit cells. A fuel flow-adjusting member is arranged in the delivery flow passage by providing a gap with respect to the inflow port. The flow-adjusting member is composed of a porous member having a predetermined thickness to allow the fuel to permeate therethrough. The flow-adjusting member functions to adjust the flow of the fuel in the delivery flow passage.
However, the fuel cell stack comprises the fuel cell units and the separators which are alternately stacked with each other. Therefore, any steps arise for each of the separators in the communication hole. The fluid such as the fuel fails to smoothly flow through the communication hole. As a result, the pressure loss is increased. For this reason, a problem is pointed out in that the fluid cannot be delivered uniformly for each of the fuel cell units. In this context, it is conceived to enlarge the opening cross-sectional area of the communication hole. However, such an arrangement enlarges the size of the stack, resulting in a problem that the entire system becomes large.
A principal object of the present invention is to provide a fuel cell stack which makes it possible to reduce the pressure loss in the communication hole and uniformly deliver the fluid to each of the fuel cell units.